Today, optical-based data storage systems are commercially competitive due to their high storage density, relatively low cost, and random access capability. Moreover, magneto-optical data storage systems offer the added flexibility of allowing an optical medium to be erased and new data written in place of the erased section. This feature grants a user the capability to reuse an optical medium many times over by erasing old data and substituting new data in place thereof.
Basically, magneto-optical recording operates in the following manner. Data is stored as a series of binary bits (i.e., 1s and 0s). A laser beam is focused onto an optical medium, usually by means of a lens assembly. Initially, the optical medium is perpendicularly magnetized. To write a "1," the laser beam is pulsed at a high power for a short duration. This raises the temperature of the optical medium to such a degree that an externally applied magnetic field reverses the direction of magnetization in the heated region. When the medium returns to its lower ambient temperature, the "domain" retains its reverse magnetization.
The domains are "erased" by using the laser to perform the same thermal process used to write the data, except that an oppositely directed external magnetic field is applied. Thereby, the domains revert back to their original magnetization.
The stored data is read from the optical medium based on the polar curve principle. This principle states that linearly polarized light, reflected from a perpendicularly magnetized medium, is rotated according to the direction of magnetization. Hence, the magnetization transitions of the domains stored on the media can be read by determining the direction of the plane of polarization of the reflected light. The same laser used to write the data is also used to generate the reflected light for reading the stored data, except that its power is reduced to avoid inadvertently writing data onto the medium.
This type of recording system has one common problem in that, typically, there is no isolation between the laser and the storage medium. Consequently, the light generated by the laser used to read the data stored on the medium is reflected from the medium and directed back at the laser. As a result of this feedback light, the laser becomes unstable. The instability interferes with the reading of data from the medium. That is to say, the instability is a form of noise.
One prior art approach to minimize the instability caused by the feedback light has been to incorporate a radio frequency (RF) modulator. FIG. 1 illustrates a typical prior art RF modulator and laser driver used in magneto-optical recording systems. A laser driver 100 is used to drive a laser diode 101. The RF modulator 102 is implemented to modulate the laser diode 101 at relatively high frequencies (approximately 300-600 MHz). By modulating the laser diode 101, instabilities due to reflected light from the medium are minimized.
However, one disadvantage associated with implementing an RF modulator is that RF energy is hard to contain and channel because it tends to radiate to the surroundings and to transmit through wiring. Hence, the modulator is enclosed within an electro-magnetic interference (EMI) shielding box 103. Inductor 104, feed-through capacitor 105, and resistor 106 are implemented as a low-pass filter to reduce the amount of RF energy being transmitted back to laser driver 100 through line 107. If the LRC 104-106 low-pass filter were not implemented, the RF energy from modulator 102 would trace back to laser driver 100 through line 107, thereby creating a D.C. offset in pre-amplifier 108. The D.C. offset can be compensated, but since RF pickup varies from one device to another, each device must be individually compensated manually. Such a process is labor intensive, costly, and time-consuming.
However, implementing an LRC 104-106 low-pass filter is disadvantageous because it limits the bandwidth of laser driver 100 in WRITE mode operations. In other words, the same low-pass filter which reduces the RF energy from tracing back to the laser driver during READ operations also acts to inhibit the bandwidth of the laser driver during WRITE operations.
In typical prior art magneto-optical recording systems, low bandwidth laser drivers were sufficient to write data onto storage media at certain locations. The sizes and boundaries of the written domains were not critical so long as the domains were adequately spaced apart and written at the proper locations. The data is read from the media by detecting the locations of each of the written domains. This scheme is known as pulse position detection (PPD). As designers strive to increase the capacity of the storage media by writing the domains closer together, the domains start to interfere with one another. In order to write the domains closely together, the boundaries of each domain need to be precisely defined to minimize interference. A higher WRITE mode bandwidth enables the laser drive to more precisely control the power of the laser. In turn, the edges of the domains can be more precisely defined, resulting in higher storage capacities.
Furthermore, magneto-optical recording systems are being developed which incorporate pulse width modulation (PWM) schemes. PWM is utilized to further increase the capacity of a given media. In PWM, data depends not only from the position of the domain but also from its width. Since information is a function of a domain's width, it is important to precisely control the rising and falling transition edges of a write pulse. Again, a high WRITE mode bandwidth is highly desirable to precisely control the pulse transition edges and, hence, the width of the domains.
One prior art method of implementing a high bandwidth WRITE mode, which also includes an RF modulator and a low-pass filter, is to output a relatively strong signal from the laser driver to drive the laser diode. However, this method is disadvantageous because the strong signal on the wire 107 leading from the laser driver 100 to the EMI shielding 103 causes serious radiated emission problems. The problem is worsened by the fact that the output from the laser driver contains a high level of harmonics due to the fast rise and fall times of the write pulses. In some instances, the emission problems are so serious as to cause the magneto-optical data storage system to fail Class "A" EMI standards set forth by the Federal Communication Commission (FCC).
Therefore, what is needed is a means for containing RF energy while, simultaneously, providing high WRITE mode bandwidth without high radiated emissions.